The present invention generally relates to electrographic imaging machines such as copiers and printers. More particularly, the present invention relates to such machines which have a preheating saddle for heating a recording medium or substrate such as paper.
In printing or copying machines, for the thermo-printing fixing of toner images on a recording substrate such as paper, thermofixing devices are used which have a preheating saddle with a fixing zone connected downstream and comprising a heated fixing roller and nip roller.
Thermofixing devices of this type are, for example, known from U.S. Pat. No. 4,147,992 or Japan-Abstract Vol. 13, No. 120, Mar. 24, 1989 (JP-A-63-292177).
It is further known from U.S. Pat. No. 4,835,573 to match the temperature of the fixing station automatically to the thickness or to the basis weight of the paper.
In the case of the known fixing devices, it has previously been assumed that it is necessary to preheat the paper very rapidly over a relatively short path, via the preheating saddle, and then to fix the toner image on the paper via the rollers. For this purpose, the heating elements are arranged in the preheating saddle in such a way that the greatest quantity of heat is emitted to the recording substrate in the region of the paper inlet of the preheating saddle and that the emitted quantity of heat is then reduced over the heating elements in the direction of the paper exit. Thus, the relatively hottest region of the saddle is the paper inlet.
However, it has appeared that a rapid heating up of the paper over a short path leads to a high loading of the paper. This loading is expressed as a deformation, an embrittlement or an ageing of the paper and as a non-uniform loss of water from the paper during passage through the fixing station. Hence, post-processing of the paper by cutting or sorting is made more difficult or there occurs a non-uniform fixing of the toner images and thus an impairment of the quality of the print.
In addition, a rapid heating up requires a high specific heating power using high-power heating elements and a complicated control system. Because of the high heating power it is therefore necessary to lift the recording substrate immediately from the saddle in the event of a printer stop, in order to prevent burning of the paper. This makes comprehensive control devices necessary, which impairs the paper handling as a whole.
In modem electrophotographic printing devices, furthermore, recording substrates of the most different widths are processed in the same machine. If the same amount of energy is fed to the saddle over the entire width, the saddle heats up severely in that region where there is no paper running, since in this region no energy is dissipated, apart from losses due to convection.
A temperature distribution of this type has considerable disadvantages. The paper is heated up non-uniformly, which leads to fluctuations in the fixing quality and can also cause paper running problems. The maximum heating saddle temperature met be reduced, since there exists the risk of overheating of the heating elements end the lifetime of the heating elements is thereby shortened. The energy losses are relatively large and the inner region of the machine is heated up unnecessarily.
In the case of thermofixing devices with a preheating saddle, the recording substrate is guided over a heated gliding surface of the saddle. Direct contact between paper and saddle is essential for a good thermal transfer between paper and saddle surface. In the case of high printing speeds and in the use of pre-folded papers or papers of non-uniform thickness, fluttering movements of the paper can occur in the region of the saddle. In consequence, the paper lifts partially off from the saddle, which impairs the thermal transfer. Also, paper contains a relatively high proportion of water, which is released during warming. The released steam can be deposited in the machine and can lead there to disturbances or to corrosion.